1. Field of the Invention
The present invention relates to measuring chromatic dispersion characteristic of a DUT (Device Under Test) such as an optical fiber, and more specifically relates to technology for measuring without being influenced by a contraction/extension of the DUT.
2. Description of the Related Art
When chromatic dispersion characteristic of a device under test (DUT) such as an optical fiber are measured, it is desirable to measure while eliminating the influence of a contraction/extension of the DUT. A technology for measuring without being influenced by the contraction/extension of the DUT is described in Publication of Japanese Patent Laid-Open No. H01-291141.
FIG. 4 shows a constitution of its measuring system. As described in FIG. 4, the measuring system is divided into a light source system 10 and a characteristics measuring system 20. A variable wavelength light source 12 in the light source system 10 changes a wavelength to generate light with a wavelength of xcexx (variable wavelength light). A fixed wavelength light source 13 fixes a wavelength to generate light with a wavelength of xcex0 (fixed wavelength light). xcex0 is a wavelength which provides the minimum chromatic dispersion in a DUT 30. The variable wavelength light and the fixed wavelength light are modulated with a frequency of f respectively by an optical modulator 15a, and an optical modulator 15b, and are composed by a multiplexer 16. The frequency f is provided by power supplies for modulating, which are omitted in the drawing.
Light composed in the multiplexer 16 enters into the DUT 30. The light transmitted through the DUT 30 enters an optical demultiplexer 21 of the characteristics measuring system 20. The optical demultiplexer 21 separates the transmitted light through the DUT 30 into light with the wavelength of xcexx and light with the wavelength of xcex0. An optical/electrical converter for measuring 22a and an optical/electrical converter for reference 22b respectively apply optical/electrical conversion to the light with the wavelength of xcexx and the light with the wavelength of xcex0, and a phase comparator 24 detects a phase difference between an output from the optical/electrical converter for measuring 22a and an output from the optical/electrical converter for reference 22b. 
The transmitted light with the wavelength of xcexx is affected by the chromatic dispersion and the contraction/extension of DUT 30. The transmitted light with the wavelength of xcex0 is affected only by the contraction/extension of DUT 30. This is because xcex0 is the wavelength which provides the minimum chromatic dispersion in DUT 30. Thus, detecting the phase difference between the transmitted light with the wavelength of xcexx and the transmitted light with the wavelength of xcex0 removes the affect of contraction/extension of DUT 30.
However, it is required to provide the optical modulator 15a and the optical modulator 15b with the same frequency for modulating. In other words, it is impossible to set the frequency for modulating in the optical modulator 15a and the frequency for modulating in the optical modulator 15b different from each other.
A purpose of the present invention is to provide an apparatus and the like for measuring the chromatic dispersion when the modulating frequency for the variable wavelength light source and the modulating frequency for the fixed wavelength light source for reference are different.
According to the present invention as described in claim 1, an optical characteristics measuring apparatus for measuring characteristics of light transmitted thorough a device under test includes: a variable wavelength light source for generating variable wavelength light; a fixed wavelength light source for generating fixed wavelength light; a variable wavelength light modulating unit for receiving a signal having a first modulating frequency, and modulating the variable wavelength light with the first modulating frequency; a fixed wavelength light modulating unit for receiving a signal having a second modulating frequency, which is different from the first modulating frequency, and modulating the fixed wavelength light with the second modulating frequency; a composite light generating unit for entering composite light composed of the variable wavelength light and the fixed wavelength light into the device under test; a wavelength component extracting unit for extracting a fixed wavelength light component and a variable wavelength light component from transmitted light transmitted from the device under test; and a phase comparing unit for measuring a true phase difference between the variable wavelength light component having the first modulating frequency and the signal having the first modulating frequency based on the variable wavelength light component, the fixed wavelength light component, the signal having the first modulating frequency, and the signal having the second modulating frequency; wherein characteristics of the device under test is obtained from the true phase difference.
With the optical characteristics measuring apparatus constituted as described above, the phase difference between the phase of variable wavelength light component and the phase of signal having the first modulating frequency includes the affect of contraction/extension and the like of the device under test. On the other hand, the phase difference between the phase of fixed wavelength light component and the phase of signal having the second modulating frequency includes only the affect of contraction/extension and the like of the device under test. Thus, it is possible to remove the affect of contraction/extension and the like of the device under test from the phase difference between the phase of variable wavelength light component and the phase of signal having the first modulating frequency by taking into account of the phase difference between the phase of fixed wavelength light component and the phase of signal having the second modulating frequency. In other words, a true phase difference is measured. Also, the first modulating frequency may be different from the second modulating frequency.
The xe2x80x9ctrue phase differencexe2x80x9d here is a phase difference when the affect of contraction/extension of device under test is removed.
The present invention as described in claim 2, is the optical characteristics measuring apparatus as claimed in claim 1, wherein the phase comparing unit is provided with: a variable wavelength light phase comparing unit for obtaining a phase difference between the variable wavelength light component and the signal having the first modulating frequency; a fixed wavelength light phase comparing unit for obtaining a phase difference between the fixed wavelength light component and the signal having the second modulating frequency; a phase difference converting unit for converting the phase difference calculated by the fixed wavelength light phase comparing unit to what corresponding to the first modulating frequency; and a true phase difference calculating unit for calculating a true phase difference from the phase difference calculated by the variable wavelength light phase comparing unit and the converted result of the phase difference converting unit.
According to the present invention as described in claim 3, the optical characteristics measuring apparatus as claimed in claim 1 further includes a characteristics calculating unit for calculating group delay or chromatic dispersion of the device under test from the true phase difference.
According to the present invention as described in claim 4, an optical characteristics measuring method for measuring characteristics of light transmitted thorough a device under test includes: a variable wavelength light generating step for generating variable wavelength light; a fixed wavelength light generating step for generating fixed wavelength light; a variable wavelength light modulating step for receiving a signal having a first modulating frequency, and modulating the variable wavelength light with the first modulating frequency; a fixed wavelength light modulating step for receiving a signal having a second modulating frequency, which is different from the first modulating frequency, and modulating the fixed wavelength light with the second modulating frequency; a composite light generating step for entering composite light composed of the variable wavelength light and the fixed wavelength light into the device under test; a wavelength component extracting step for extracting a fixed wavelength light component and a variable wavelength light component from transmitted light transmitted from the device under test; and a phase comparing step for measuring a true phase difference between the variable wavelength light component having the first modulating frequency and the signal having the first modulating frequency based on the variable wavelength light component, the fixed wavelength light component, the signal having the first modulating frequency, and the signal having the second modulating frequency; wherein characteristics of the device under test is obtained from the true phase difference.
The present invention as described in claim 5, is a computer-readable medium having a program of instructions for execution by the computer to perform an optical characteristics measuring process for measuring characteristics of light transmitted thorough a device under test. The optical characteristics measuring process includes: a variable wavelength light generating processing for generating variable wavelength light; a fixed wavelength light generating processing for generating fixed wavelength light; a variable wavelength light modulating processing for receiving a signal having a first modulating frequency, and modulating the variable wavelength light with the first modulating frequency; a fixed wavelength light modulating processing for receiving a signal having a second modulating frequency, which is different from the first modulating frequency, and modulating the fixed wavelength light with the second modulating frequency; a composite light generating processing for entering composite light composed of the variable wavelength light and the fixed wavelength light into the device under test; a wavelength component extracting processing for extracting a fixed wavelength light component and a variable wavelength light component from transmitted light transmitted from the device under test; and a phase comparing processing for measuring a true phase difference between the variable wavelength light component having the first modulating frequency and the signal having the first modulating frequency based on the variable wavelength light component, the fixed wavelength light component, the signal having the first modulating frequency, and the signal having the second modulating frequency; wherein characteristics of the device under test is obtained from the true phase difference.